As is well-recognized in the art, glass-ceramic bodies are prepared through the heat treatment of precursor glass bodies. Thus, the method for producing glass-ceramic articles contemplates three general steps: first, a batch for a glass of a particular composition is melted; second, that melt is cooled to a temperature below the transformation range thereof and, simultaneously, a glass body of a desired geometry is shaped therefrom; and, third, that glass body is exposed to temperatures above the transformation range in a controlled manner to cause crystallization in situ of the glass. Often, the third step will comprise a two-stage heat treatment in which the parent glass body is initially heated to a temperature within or somewhat above the transformation range for a sufficient length of time to generate nuclei; and thereafter the tempreature is raised, frequently above the softening point of the glass, to cause the growth of crystals on the nuclei. This two-stage heat treatment yields glass-ceramic bodies which typically are more highly crystalline and the crystals more uniformly fine-grained. Because the growth of crystals takes place in situ, glass-ceramic bodies are non-porous and free from voids.
Inasmuch as glass-ceramic articles are normally highly crystalline, i.e., at least 50% by volume crystalline, they are usually mechanically stronger than the parent glass bodies from which they are derived. For example, whereas annealed glass articles customarily demonstrate moduli of rupture over the range of about 5000-10,000 psi, glass-ceramic articles frequently exhibit moduli of rupture varying over the interval of about 10,000-20,000 psi.
Although a glass-ceramic article often displays a modulus of rupture twice that of its parent glass, considerable effort has been expended over the years to significantly enhance the mechanical strengths of glass-ceramic articles. Furthermore, whereas glass-ceramic articles often manifest smooth, glass-like surfaces, there has been the desire, especially where the articles will be used as tableware, to apply decorations thereto. Also, in certain instances it has been found desirable/necessary to protect the surface of glass-ceramic articles from attack by chemical agents.
One means for accomplishing those three objectives has involved the use of glazes or enamels. Glazes are generally deemed to be clear glasses; colored glazes are considered to be clear glasses with colorants dissolved therein; and enamels are defined as glazes having pigments suspended therein to impart color and/or opacity thereto. Both glazes and enamels are customarily applied to the surface of a glass-ceramic article in the form of very finely-divided particles or powder, termed by the art "frit", suspended in a volatile vehicle to yield a paste or slurry. The frit is subsequently fired to first remove the vehicle and thereafter at a higher temperature to fuse the particles together to form a strongly-adherent, continuous film on the surface of the article.
A frit must demonstrate two fundamental characteristics to be operable as a glaze or enamel for a particular glass-ceramic substrate: (a) the firing or maturing temperature thereof, i.e., the temperature at which the frit will flow sufficiently to produce a smooth uniform coating on the glass-ceramic, must be low enough to avoid thermal deformation of the glass-ceramic article being coated; and (b) the coefficient of thermal expansion thereof must be compatible with that of the glass-ceramic to inhibit crazing and/or spalling of the coating. Preferably, the coefficient of thermal expansion of the frit will be somewhat lower than that of the glass-ceramic such that the resultant coating is placed under compressive stress which, in turn, enhances the overall mechanical strength of the final product.
As can be appreciated, the sale of products designed for use as tableware is strongly influenced by the visual appearance thereof. Therefore, the use of enamels and glazes to alter and/or improve the aesthetic qualities of such products has been extensive. Moreover, the use of certain enamels and glazes has preserved the attractive surface finishes developed by inhibiting food staining and chemical attack from such items as foods, detergents, and other cleaning agents.
The use of glass-ceramic bodies in tableware applications had its genesis in U.S. Pat. No. 3,201,266. That patent disclosed glass-ceramic articles in the Na.sub.2 O--BaO--Al.sub.2 O.sub.3 --SiO.sub.2 --TiO.sub.2 composition containing crystals of nepheline and celsian or hexacelsian. Those articles exhibited coefficients of thermal expansion ranging between about 75-110.times.10.sup.-7 /.degree. C.
U.S. Pat. No. 3,384,508 describes glazes especially designed for use with glass-ceramic bodies containing nepheline as a primary crystal phase. Those glazes consisted essentially, in weight percent on the oxide basis, of:
Na.sub.2 O: 1.5-16 PA1 K.sub.2 O: 0-8 PA1 Na.sub.2 O--K.sub.2 O: 1.5-16 PA1 Al.sub.2 O.sub.3 :3-10 PA1 B.sub.2 O.sub.3 : 4-19 PA1 SiO.sub.2 : 40-70 PA1 MgO: 0-9 PA1 CaO: 0-12 PA1 PbO: 0-40 PA1 ZrO.sub.2 : 0-5 PA1 CdO: 0-5 PA1 CaF.sub.2 : 0-8.5 PA1 ZnO: 0-2. PA1 SiO.sub.2 : 50-70 PA1 CaO: 4-15 PA1 MgO: 8-25 PA1 F: 3-8 PA1 Na.sub.2 O: 2-9 PA1 K.sub.2 O: 2-12 PA1 Li.sub.2 O: 0-3 PA1 Al.sub.2 O.sub.3 : 0-7. PA1 SiO.sub.2 : 46-50 PA1 Al.sub.2 O.sub.3 : 5-8 PA1 ZrO.sub.2 : 0.3-1.0 PA1 Li.sub.2 O: 1-2 PA1 Na.sub.2 O: 2-3.5 PA1 K.sub.2 O: 1-2 PA1 CaO: 1-4 PA1 ZnO: 2-6 PA1 SrO: 2-6 PA1 B.sub.2 O.sub.3 : 9-11 PA1 PbO: 20-25. PA1 SiO.sub.2 : 37-49 PA1 Al.sub.2 O.sub.3 : 4-7 PA1 B.sub.2 O.sub.3 : 6.5-11 PA1 ZrO.sub.2 : 0-1.75 PA1 Na.sub.2 O: 1.75-3.5 PA1 K.sub.2 O: 0.75-2.5 PA1 PbO: 21-32 PA1 CdO: 0-0.75 PA1 CaO: 1-9 PA1 TiO.sub.2 : 0-0.35 PA1 F: 0-0.6. PA1 SiO.sub.2 : 47-49 PA1 Al.sub.2 O.sub.3 : 4.5-5.5 PA1 B.sub.2 O.sub.3 : 6.5-7.5 PA1 ZrO.sub.2 : 0.75-1.75 PA1 Na.sub.2 O:1.75-2.75 PA1 K.sub.2 O: 0.75-1.5 PA1 PbO: 27.5-29.5 PA1 CdO: 0.2-0.5 PA1 CaO: 5.75-7 PA1 TiO.sub.2 : 0.05-0.35 PA1 F: 0.2-0.6.
Those glazes were operable with the glass-ceramics of U.S. Pat. No. 3,201,266 and reacted with the surface of the glass-ceramic body to form an intermediate layer between the glass-ceramic and the glaze consisting of crystals extending into the glaze and interlocking therewith.
Recently, glass-ceramic bodies have been developed which not only exhibit moduli of rupture in excess of 20,000 psi but also demonstrate high intrinsic toughness against fracture propagation, thereby rendering them especially suitable for the production of tableware. As disclosed in U.S. Pat. No. 4,467,039, those glass-ceramics contain potassium fluorrichterite as the predominant crystal phase and consist essentially, in weight percent on the oxide basis, of:
Canasite may be present as a second fluorosilicate phase but, for use in tableware application, having the crystal phase composed either substantially solely of potassium fluorrichterite or of potassium fluorrichterite with a minor amount of cristobalite is preferred.
Whereas the glazes of U.S. Pat. No. 3,384,508 can provide excellent gloss, exhibit very low release of heavy metals, e.g., lead and cadmium, and display good resistance to chemical attack by foods, detergents, etc., the glazes disclosed in that patent demonstrating the best performance require firing temperatures in the vicinity of 1050.degree. C. The glass-ceramics of U.S. Pat. No. 4,467,039, however, are not as refractory as those of U.S. Pat. No. 3,201,266. Consequently, the glazes of U.S. Pat. No. 3,384,508 exhibiting the most desirable properties cannot be used with the glass-ceramics of U.S. Pat. No. 4,467,039, since the firing temperatures necessary for maturing those glazes cause thermal deformation and slumping of those glass-ceramics.
Therefore, the principal objective of the present invention was to develop glazes operable with the glass-ceramics of U.S. Pat. No. 4,467,039, while having properties rendering them particularly suitable for tableware applications.